A number of attempts have been made to design and fabricate high precision sensing systems. In one known type of system, an encoder is attached to a movable member, and the position of the member is determined by interrogating the encoder with an optical or electrical signal. The member may be one that rotates, in which case the encoder may comprise a disk that rotates with the member, or may be linearly movable, in which case the encoder moves linearly along with the member.
In an analog system, the encoder typically includes a track that has a continuously variable property, such as a continuously variable optical transmission or reflection coefficient, or a continuously variable electrical resistance. In one known optical arrangement, an optical signal is transmitted to the encoder by an optical fiber, passes through the variable density track, and is then transmitted back to a suitable detector by an optical fiber. The optical attenuation of the fiber-optic links is an unknown and variable factor that must be determined before the position of the encoder can be inferred solely from an end-to-end attenuation measurement. In a so-called two-wavelength referenced system, the link attenuation is measured by transmitting two optical signals having different wavelengths, and by designing the sensor such that only one signal is attenuated as a function of the encoder position. Attempts have been made to fabricate systems of this type that are capable of high-precision. However, it has been found that the performance is limited by differential bending loss with wavelength in the optical fibers, and differential loss due to different mode structures caused by different signal launching conditions. The stability of two wavelength referenced systems is generally accepted to be about 1 percent.
In prior digital encoding systems, the encoder includes a number of parallel coded tracks, each of which represents a specific bit in a binary word. Each track comprises a series of elements or positions, each of which has a property that can assume one of two states, such as transmitting/nontransmitting or reflecting/nonreflecting in optical systems, or conducting/nonconducting or high voltage/low voltage in electrical systems. For each position of the digital encoder, the tracks will present a different combination of elements, and therefore a different binary word, to the detection system. The precision is limited only by the highest achievable element density of the least significant track. In an optical system, wavelength division multiplexing (WDM) may be used to interrogate each track with light in a different wavelength range. This arrangement permits optical signals to be coupled to and from the sensor along single fiber-optic cables.
WDM digital encoding systems have been described that use a combination of a GRIN rod lens, a prism, and a diffraction grating. Such a system is optically complicated and inefficient, and requires a broad band source for operation. A fundamental problem, common to all WDM encoders that use a diffraction grating as the dispersive element, is that a reasonably well-collimated beam is required if reasonable resolution and channel width are to be obtained. In particular, one dimension of the beam should be no wider than the dimensions of the elements of the least significant track along the length of the track. This limits the resolution to the fiber core diameter. Smaller diameter fibers mean that less light can be launched into the fiber, thereby reducing the signal-to-noise ratio. A compromise must therefore be made between resolution, optical efficiency, system losses, and the physical size of the sensor. Similar considerations apply to the use of interference filters to demultiplex the channels.